The present invention relates, in general, to infusion and transfusion apparatus, and more particularly, to flow control devices for infusion and transfusion apparatus.
Control of the flow rate of fluid in infusion and transfusion apparatus is one very important consideration in constructing such apparatus. The prior art discloses various means for effecting such flow control. Manual flow regulators are quite common in the prior art, and such regulators often use clamps or other such manually operated devices to control flow. Such manually controlled regulators are convenient, but do not provide the flow control accuracy required in modern medicine. Recently, electronically controlled regulators have been utilized to control flow in infusion and transfusion systems. Examples of such electronic systems are disclosed in LeFevre, et al. U.S. Pat. No. 4,038,981 and Burke, et al. U.S. Pat. No. 4,038,982. These patents have a complete disclosure of an electronic system, and reference is made thereto for such description. Such electronic regulators produce exceptionally accurate and reliable control of the rate with which fluid is administered to a patient.
It is noted that infusion and transfusion systems utilize a means for connecting tubes to the fluid source which comprises at least two main elements, a piercing device which is attached directly to the source of fluid, and a drip chamber which transfers the fluid from the piercing device to the tubing via tubing connectors. A degree of mechanical control of the fluid flow in an infusion and transfusion apparatus can be exercised in the drip chamber portion of the apparatus. Flow control means, such as metering orifices and the like, are often used.
It has been observed that air in a drip chamber acts as a "cushion" and exerts a strong influence on fluid flowing into the chamber through an inlet port means in that chamber. The air located adjacent the inlet port means determines the back pressure on that port, which, along with the pressure of the fluid upstream of the port, determines the pressure differential established across the port. The pressure differential, in turn, determines the amount and character of the flow through the port means. Thus, if the fluid pressure existing upstream of the inlet port is fairly constant, flow through the port responds in rate and character to changes in the back pressure, or that pressure in the drip chamber existing immediately adjacent the port. Such pressure changes generally only occur as a result of influences exerted on the drip chamber from the system downstream of the drip chamber. Thus, flow demands made on the solution source from the system are transmitted to the source via pressure disturbances in the drip chamber. It is noted that propagation of a pressure wave through a medium is subject to damping effects, and thus the flow demands of the overall system are subject to being damped by the air cushion existing in the drip chamber, and accordingly, these flow demands are not accurately and precisely transmitted to the fluid source by the drip chamber, thereby resulting in the above-discussed inaccuracies which render known drip chamber configurations inadequate for use in conjunction with the extremely accurate and precise electronic flow control systems.
This phenomenon can be stated in hydraulic terms by viewing the cushion of air in the drip chamber as a damping means which tends to damp out pressure perturbations transmitted through the drip chamber. During administration of fluid to a patient, fluid is withdrawn from the source in response to pressure perturbations transmitted through the drip chamber from the tubing adaptor. Thus, a large air cushion with the attendant pressure perturbation damping effects thereof may tend to vitiate the precision with which the drip chamber fluid feed mechanism responds to the flow demands placed thereon by the overall system.
In the referenced LeFevre and Burke patents, for example, the air volume in the drip chamber below the metering orifice is approximately 2 cc which has often resulted in double or triple drops from a 60 drop per milliliter orifice during a single cycle of the electronic flow control apparatus. In the referenced patents, the flow control mechanism utilizes an electronic timing device which is triggered by a droplet traversing the drip chamber. Such multiple drop formations may therefore cause electronic control to mismanage the fluid flow through the apparatus or to malfunction during the control cycle. Therefore, while the presently known drip chamber formations and configurations are sufficiently accurate for most applications, the extreme precision of the electronic flow controls, such as those controls disclosed in the referenced patents, requires much faster response and greater exactness than can be provided by known drip chambers.
A further function of the drip chamber portion of the infusion and transfusion apparatus is in priming the system, that is, initiating flow from the source of fluid. This function is generally performed by manually manipulating a flexible portion of the drip chamber to create a suction on the fluid source. Therefore, manufacture of a drip chamber has several constraints placed thereon to account for this function. The flexible portion of the drip chamber must be large enough to be easily manipulated during the priming operation, yet the overall drip chamber must be small enough to prevent generation of an air cushion adjacent the inlet port, which air cushion is of sufficient size to produce multiple drops, as above discussed. Furthermore, the pump chamber can serve as a reservoir of fluid to prevent interruption of flow during the initial moments after an empty bottle situation, and should be manufactured to account for this function as well.
The fore end of the drip chamber is assembled with a piercing device, and thus should be somewhat rigid to facilitate easy coupling of the drip chamber and the piercing device assembly.
Therefore, an ideal drip chamber should monitor flow from the source of fluid with an accuracy commensurate with electronic monitoring systems, yet be amenable to efficient manufacture and use.